As disclosed in the copending U.S. patent applications Ser. Nos. 418,005, filed Nov. 21, 1973 now Pat. No. 3,907,630 now dated 4/23/75, and 504,739, filed Sept. 10, 1974 now Pat. No. 3,977,540 date 6/29/76, respectively, the essential condition in the manufacture of fiberboard according to the wet method, which renders possible completely to close the backwater system and to produce the fiber suspension with backwater, i.e., white water only, is that the fibrous material fed into the process flow must have a content of dry substance which is substantially higher than that of the wet sheet prior to the final drying thereof by evaporation of water. Depending on the density which is to be imparted to the fiberboard, the final drying can be effected under simultaneous mechanical compression or without such compression.
In the manufacture of hardboard, for example the wet sheet is subjected to so high a mechanical pressure that the compressed wet sheet acquires a dry content of 50 to 55 percent before the final drying operation is carried out. In order to obtain this dry content of the compressed sheet, the fiber material discharged from a disintegrating or grinding apparatus, hereinafter referred to by the name "defibrator," must have a dry substance content of at least 50 to 55 percent. Normally, the content must be kept between 55 and 70 percent in order to permit a certain quantity of water to enter the process without causing any danger of discharge of backwater into the drainage system. When manufacturing insulation fiberboard, in which case the dry content of the wet sheet amounts to between 40 and 50 percent, a dry content of the discharged fiber material of between 50 and 55 percent is sufficient.
Utilization of water as sealing agent at the places of passage of the shaft in the defibrator must be limited or completely eliminated, e.g., by utilization of steam.
Fiber material suited for the process according to the invention may be produced from any kind of lignocellulose-containing fibrous material, which has been disintegrated in a suitable manner, e.g., wood in the form of chips or sawdust, or straw, or bagasse etc., which hereinafter is generally referred to as "chips".
The defibration is assumed to be effected in a steam atmosphere at atmospheric pressure or increased pressure up to 15 atmosphere excess pressure, corresponding to a temperature range between 100.degree. and 200.degree. C.
In the defibration of wood chips which have been thoroughly heated in saturated steam, usually about 250 KWh are consumed per ton chips, calculated as bone dry, if the defibration is performed at a temperature 160.degree. and 170.degree. C. At lower temperatures, an increased consumption of energy must be calculated with. The supplied electrical energy is converted almost quantitively into thermal energy in the form of steam. The heat production is concentrated in the grinding zone, and in order to avoid local overheating of the fiber material, a sufficient amount of water must be present, and a grinding produce concentration in the grinding zone, which is in excess of 60 percent is not advisable.
Assuming that the defibration is effected at a temperature of 165.degree. C and that the chips have a dry content of 50 percent and a temperature of 5.degree. C when introduced into the defibrator, the concentration of the grinding produce within the defibrator will be about 47 percent and the concentration of the pulp after blowing it out to atmospheric pressure will be about 51 percent, provided the chips have been heated up to 165.degree. C by means of steam and the defibrating energy amounts to 240 KWh per ton bone dry substance and that no dilution has occurred through sealing water or moisture entering the steam. If the dry content of the chips is 55 percent, the pulp concentration under conditions otherwise the same will be 54 and 57.5 percent for the grinding produce and the discharged pulp, respectively. If the dry content of the chips is lower, e.g., 45 percent, the concentration values will be, respectively, 42 and 45.5 percent. Normally, however, a dilution of the pulp will occur due to entering water, and in practice one must calculate with lower pulp concentrations than those stated here.
The fact that the pulp concentration subsequent to defibration, nevertheless becomes so high, in spite of water added partly as moisture in the chips and partly as condensed steam, is due to the generation of heat in the grinding zone, whereby a quantity of water corresponding to the heat generation is evaporated.
In the manufacture of insulation fiberboard, in which case the wet sheet prior to the drying has a dry content of between 40 and 50 percent one should, under favorable conditions, be able to defibrate chips having a dry content of 50 percent.